prime number array from number java

Question

I'm trying to make a helper method that starts at a number, startLook, inclusive, and returns the next num number of primes from that number. Here is my code:

public int[] nPrimes(int num, int startLook) {
    int y = startLook;
    int x = 2;
    int[] c = new int[num];
    int d = 0;
    while (x <= y/2) {
        if (y % x == 0) {
            x++;
            continue;
        }
        if (y % x != 0) {
            c[d]=y;
            d++;
        }
        x++;
    }
    return c;
}

This results in ArrayIndexOutOfBoundsException and not registering primes. What am I doing wrong? Thanks in advance.

You should trace through your code a bit (print out some values or similar) there's some pretty serious issues with your logic here. In particular, not every number that isn't divisible a given `x` is a prime. — CollinD, Nov 12 '16 at 21:36
Oh, so I should switch the is equal and is not equal? Thanks, CollinD! And thanks, Fredrik! I didn't know that! — Not a presence, Nov 12 '16 at 21:38
You never check the boundary of the array c, d will go out of bounds. — Fredrik Lundvall, Nov 12 '16 at 21:38

score 0 · Answer 1 · answered Nov 12 '16 at 21:39

You're getting ArrayIndexOutOfBoundsException error because in your second if statement d is incremented and you never check if it reached num-1 (last possible index for your c array).

You should consider changing your while loop condition to something checking d value.

prime number array from number java

1 Answers1